1. Field of the Invention
The invention relates to polyvinylchloride (xe2x80x9cPVCxe2x80x9d) slurry being stripped in a column, or fractionation column, to remove residual vinyl chloride, and the PVC slurry passes over a plurality of slurry trays, while a vapor stream is directly injected into the bottom of the column.
2. Description of Related Art
In the production of PVC resin, vinyl chloride monomer is removed from a slurry of vinyl chloride resin by blowing, or passing, steam into a slurry of the resin while the resin flows down through the column, thus stripping off remaining monomer together with generated water vapor, as is known in the art. The fractionation tower, or column, or plate tower, as they are called in the art, contains a plurality of trays over which the slurry passes, beginning at the top of the tower until the slurry exits the bottom of the tower.
A major problem in the production of the PVC resin is that the PVC resin can discolor if it spends an extended period of time in the column. The solid particles can settle out on the trays and stay in the column exposed to elevated temperatures, at which time the particles can discolor. Whenever the slurry feed to the column stops, or the column""s operation is upset, the discolored particles can exit the column and contaminate the product. A disadvantage in many tray columns presently being used is that the velocity of the slurry is not sufficient to maintain the solid resin particles in suspension at all times.
Accordingly, it would be advantageous to have a slurry tray and a slurry tray assembly for use in fractionation towers wherein the velocity of the slurry is maintained, so that the solids will be maintained in suspension at all times as the slurry passes through the multiple trays.
In accordance with the invention, the foregoing advantage have been achieved through the present slurry tray and slurry tray assembly, or plate tower, for use in fractionation towers. The slurry trays of the present invention maintain a plug flow regime, whereby all the solid particles are exposed to the same residence time and temperature, within the column, as all other particles. Thus, discoloration of the PVC resin particles does not occur.
The slurry trays are designed with partitions, or baffles, that guide, or force the slurry to flow in channels formed upon each individual slurry tray. The channels are designed whereby the velocity of the flow is always greater than the velocity required to maintain the solids in suspension. Weirs are provided that have holes formed in the bottom of the weirs, so that the solid resin particles will not be trapped at the bottom of the weir. The holes, or openings, provide passageways through which the solid particles can pass, thus constantly flowing from tray to tray, and eventually out of the fractionation, or distillation, column. The openings provide for the solid particles not becoming trapped upon a particular tray and not passing to the next lower level, or next lower tray, in the column.
The trays of the present invention are designed to fit into a standard distillation column, or fractionation tower, or any similar structure. In each embodiment of the present invention, the trays can be provided with directional valves, which are pointed in the direction of the flow of the slurry along the tray, which assists in keeping the solid particles in suspension. The tray preferably has no horizontal surfaces on the supporting structures where solids can accumulate.
The baffle disposed upon the trays preferably channels the flow of the slurry so as to maintain the velocity of the slurry at above approximately one foot per second. This preferred velocity ensures plug flow velocity so that each particle is exposed to the same residence time and the same temperature history for each embodiment of the present invention.
The holes in the lower portion of the weirs preferably pass approximately one-fourth of the minimum flow into a downcomer and to the tray disposed below each weir. Thus, no solid particles will be trapped at the bottom of the weir and there is always a constant flushing of the weir along the bottom of the weir.
The downcomer in the column does not have any dead spaces, whereby plug flow preferably always occurs in the downcomer. The downcomer of each tray provides minimum space between the weir and the wall of the column, but also provides a large enough volume and weir length to ensure proper operation of the downcomer.
A feed nozzle is provided above the topmost slurry tray, and the feed nozzle is configured to provide no dead spaces and constant plug flow occurring in the topmost downcomer. Mini-jet tabs can also be provided in the surface of each tray to provide extra push, or velocity, to the slurry in low velocity regions of the trays. The height of the weirs of each tray is adjustable to increase, or decrease, the residence time of the slurry in the column.
The slurry tray and slurry tray assembly of the present invention are particularly useful in stripping slurry streams containing solids in concentrations up to thirty-five percent; however, these trays can also be useful for stripping any substance, other than PVC slurries, that has high solids contents.
The present invention also advantageously provides a method of removing residual vinyl chloride monomer from a PVC resin slurry so that the PVC resin is not discolored. The method generally includes supplying a PVC resin slurry to a slurry tray in a plate tower. The PVC resin slurry is allowed to flow through partitioned channels downwardly through a plurality of apertures, or openings, within the slurry tray and a downcomer of the slurry tray to a next slurry tray. The PVC resin slurry preferably travels at a velocity greater than that required to maintain solids contained within the PVC resin slurry in liquid suspension.
A vapor stream is also supplied to the plate tower preferably at a lower feed location than the PVC resin slurry is supplied. The vapor rises through the openings within the slurry tray countercurrent to the PVC resin slurry flowing down through the openings. The vapor is preferably steam, but air or another suitable vapor that can remove vinyl chloride monomer from PVC resin slurry can be used and will be known to those skilled in the art.
Solids that are contained within the PVC resin slurry are removed by allowing the solids retained at the bottom of the weir to continuously flow downward through the plate tower through a plurality of holes located within a weir of each slurry tray. The solids continually travel down the tower until the solids reach the tower bottom. In addition to the solids flowing through the weir holes, a portion of a minimum downcomer flow that is required to prevent flooding upon each slurry tray also flows through the weir holes.